SubSurface-Mirror Problem
The problem you are facing could be that the middle of your bumper model, where the mirroring starts, isn't completely flat along the x-axis. You can fix this by selecting the vertices, scaling and then pressing X and 0. 

After flattening the vertices, go to the properties panel by hitting N and put the vertice group to 0 on the x-axis.

Here are some pictures:


The situation


Select these vertices


Scale to 0 on the x axis


X co-ordinate to 0